What is the difference between proportional control valve and servo valves?

Please be elaborateWhat is the difference between proportional control valve and servo valves?
Mechanically, the valve itself may not be any different, but the (electronic) control systems certainly do vary. Servo implies closed-loop aka feedback control. ';proportional'; may suggest open-loop (no feedback), although one could in theory use a proportional valve as part of a servo valve system.





Usually with a valve you're trying to control the flow rate.





Typically proportional valves have a lot of hysteresis. So the amount that the valve opens is not just a function of the voltage applied to the valve, it is also a function of the direction that the valve moved to get to that position. It's also a function of the absolute pressure of the fluid supply.





To accurately meter flow, you need a method of sensing the flow rate. This value minus the target flow rate gives an error signal. Then a feedback control algorithm, such as a PID loop, uses information in the error signal to adjust the voltage on the valve, until eventually the target flow rate is reached.